In many circuits, particularly amplifiers, short-circuit protection is necessary to limit the output drive in order to prevent overheating of the output device when it is short-circuiting or running out of control. Ideally, short-circuit protection should not affect the otherwise normal operation of the circuit or the performance of its parameters. Just before a circuit's short-circuit protection kicks in, it typically causes an offset voltage to be seen at the input.
FIG. 1 is a graph 100 illustrating load current versus offset voltage in a circuit employing short-circuit protection. The solid line represents the relationship seen in a conventional short-circuit protection circuit. The dashed line represents the ideal case, where no offset is seen until the load current reaches a threshold value (IT). At IT, an ideal protection circuit then turns on sharply, which is seen as an instant change of the offset voltage.
FIG. 2 illustrates a conventional circuit 200 for implementing short-circuit protection. In circuit 200, Qout drives the output current. As the output current increases, the voltage drop across Rsense also increases. When the output current increases to a certain amount (I1, see FIG. 1), the voltage drop across Rsense will cause Qsense to start to turn on and steal some of the base current of Qout supplied by the current source 210. However, Qsense turns on gradually rather than sharply (i.e., not a digital on/off). In other words, circuit 200 operates such that Qsense turns on early so that it can be completely turned on at the point where the output current hits the short-circuit threshold value (IT, see FIG. 1). During this intermediate phase between I1 and IT, Qout is still able to supply the required current, but Qsense is already starting to steal some of its base current, and the gradually increasing offset voltage is seen by the input stage (not shown). This effect shown by the solid line in FIG. 1. This type of soft change is undesirable in high precision applications.